Molten metal resistant composite coatings

ABSTRACT

Composite coating materials comprising a hard carbide phase and a metallic binder that are resistant to molten metals such as aluminum are disclosed. The hard carbide phase of the composite coatings may comprise tungsten carbide, and the metallic binder may comprise a nickel-based alloy. A thin oxide layer comprising oxides of the binder metal may be provided on the surface of the composite coating. The composite coatings exhibit desirable non-wetting behavior when exposed to molten metals.

FIELD OF THE INVENTION

The present invention relates to molten metal resistant coatings, and more particularly relates to composite coatings deposited on articles such as tools that are exposed to molten aluminum.

BACKGROUND INFORMATION

Metallic parts in contact with molten aluminum typically are worn away quickly due to severe aluminum casting and smelting environments. Parts can react with molten aluminum at typical service temperatures ranging from 500° C. to 1,000° C., and the service lives of parts are relatively short. The low wear resistance of these parts makes it necessary to replace them frequently, which increases costs. The reaction of metallic parts with molten-aluminum can also cause quality problems. For example, steel components may dissolve in molten aluminum and contaminate the final product. A need exists for improved materials capable of withstanding such harsh conditions.

SUMMARY OF THE INVENTION

The present invention provides composite coating materials comprising a hard carbide phase and a metallic binder phase that are resistant to molten metals such as aluminum. The composite coating materials exhibit desirable non-wetting behavior when exposed to molten metals.

An aspect of the present invention is to provide a molten metal resistant composite coating comprising a hard carbide phase and a metallic binder comprising Ni and Cr.

Another aspect of the present invention is to provide a tool capable of withstanding exposure to molten metal comprising a substrate and a composite coating over at least a portion of the substrate, wherein the composite coating comprises a hard carbide phase and a metallic binder phase.

A further aspect of the present invention is to provide a method of coating a substrate comprising depositing a composite coating on at least a portion of the substrate, wherein the composite coating comprises a base layer comprising a hard carbide phase and a metallic binder phase.

These and other aspects of the present invention will be more apparent from the following description.

DETAILED DESCRIPTION

In accordance with embodiments of the present invention, a composite coating is applied on metallic parts as a wear-resistant coating to improve the service life of the parts. For example, the coatings can be applied to metal parts such as plungers that are exposed to molten aluminum or other metals during use. The composite coating comprises hard carbide particles and a metal binder. The carbides in the coating comprise at least 40 weight percent of the coating. In one embodiment, the hard carbide typically comprises from 40 to 80 weight percent of the composite, for example, from 60 to 70 weight percent of the composite. The metal binder may typically comprise up to 60 weight percent of the composite, for example, from 20 to 60 weight percent or from 30 to 40 weight percent of the composite. The composite coatings may be deposited on substrates by suitable techniques such as conventional plasma transferred arc (PTA) welding techniques.

The hard carbide phase may comprise hard carbides such as tungsten carbide. In certain embodiments, the tungsten carbide WC_(x) may comprise WC, W₂C, eutectic of WC and W₂C and combinations thereof. The hard carbides can be irregular, angular, rounded, or spherical in shape. In certain embodiments, 90 percent or more of the carbide particles have particle sizes from 70 to 180 microns.

The metal binder phase may comprise metal alloys such as nickel-based and cobalt-based alloys. In certain embodiments, the metal binder may comprise Ni, Cr, Si, Fe and/or B. In one embodiment, the metal binder comprises a nickel-based alloy, such as an alloy comprising from 60 to 90 weight percent nickel. The nickel-based binder metal may comprise more than 5 weight percent chromium, typically from 7 to 20 weight percent of the binder metal. In another embodiment, the binder metal is a nickel-based alloy comprising from 60 to 90 weight percent nickel, from 7 to 20 weight percent chromium, from 1 to 5 weight percent silicon, from 2 to 5 weight percent iron, from 1 to 4 weight percent boron, and incidental impurities. In certain embodiments, nickel-based alloys with a hardness range of HRC 33-50 and a chromium content of from 7 to 20 weight percent are chosen so that the resulting deposited powder has desired characteristics of weldability, toughness and non-wetting of molten metal such as aluminum.

In one embodiment, the surface of a plunger tip used in aluminum casting processes is coated with the composite coating using PTA techniques. Powders of the hard carbide and binder metal may be mixed and then deposited onto a substrate via known PTA techniques. A requirement for this application is that the plunger tip should not stick to the aluminum after the casting process. Thus, the molten aluminum should not wet the protective composite coating of the plunger. Such non-wetting characteristics may be achieved by controlling the composition of the composite coatings.

In certain embodiments, the composite coating may be flame-oxidized to form an oxidation layer on the coating surface after the PTA coating process. The oxidation layer may comprise oxides of the binder metals, for example, nickel oxides, chromium oxides, silicon oxides, iron oxides and/or boron oxides. The oxidation layer provides non-wetting characteristics to avoid reaction with the molten metals such as aluminum. The oxide layer may be formed by known flame oxidation techniques, such as utilizing an oxidizing flame from an oxy-acetylene torch with the composite after the PTA welding process. The oxidation layer can be repetitively formed on the composite coating surface when the surface is exposed to molten metal during use.

The composite coating may have a typical thickness of at least 0.5 mm, for example, from 0.5 to 10 mm, or from 1 to 8 mm, or from 2 to 5 mm. The oxide coating may have a typical coating thickness from 1 to 100 microns, for example, less than 50 microns, or less than 30 microns.

The substrates to which the composite coatings of the present invention are applied may be metal substrates, such as steel or other iron-containing alloys. During molten aluminum smelting and casting operations, it has been found that plunger tips and other tools of bare metal will react with the molten aluminum and cause sticking of the tool to the aluminum cast product if the protective composite coatings of the present invention are not used. The metal tool surfaces can be worn away quickly due to the reaction of the bare metal tool with molten aluminum and abrasion caused by the molten aluminum. The composite coatings of the present invention can withstand such severe conditions in molten aluminum. The composite coatings provide relatively thick protection layers on tools such as plunger tips that provide excellent wear resistance. For example, service life increases of at least 300 percent have been achieved in plunger tips used in molten aluminum casting operations in comparison with conventional plunger tips.

The following example is intended to illustrate various aspects of an embodiment of the present invention, and is not intended to limit the scope of the present invention.

Example

A composite coating was made by mixing 65 weight percent WC_(x) powder comprising a combination of WC and W₂C, 26 weight percent of a first nickel-based binder alloy powder, and 9 weight percent of a second nickel-based binder alloy powder, followed by PTA welding of the powder mixture onto a steel substrate. Greater than 90 percent of the WC, particles were between 74 and 177 microns in diameter. The first nickel-based binder alloy comprised about 78 weight percent nickel, about 11 weight percent chromium, about 3 weight percent silicon, about 3.5 weight percent iron, and about 2.5 weight percent boron. The first nickel-based binder alloy powder had particles sized such that more than 90 percent of the particles fall within the range of 44 microns to 105 microns. Such an alloy may be a self-fluxing powder which provides relatively soft coatings with good wetting qualities. The second nickel-based binder alloy comprised about 84.74 weight percent nickel, about 7.43 weight percent chromium, about 3.52 weight percent silicon, about 2.48 weight percent iron, about 1.55 weight percent boron, and about 0.25 weight percent carbon. The second nickel-based binder alloy powder comprised particles sized such that more than 90 percent of the particles fall within the range of 45 microns to 150 microns. The combination of the first nickel-based binder alloy and second nickel-based binder alloy results in a nickel-based alloy about 10.1 weight percent chromium.

The powders were mixed and then welded onto the steel substrate via a conventional PTA welding technique. The composite coating was deposited at a thickness of about 4 mm. A thin oxidized layer was then formed on the surface of the composite coating by applying an oxidizing flame from an oxy-acetylene welding torch using a #5 tip to the exterior portion of the composite coating for sufficient time to create the desired coating. The resultant composite coating with the oxidized surface is capable of withstanding exposure to molten aluminum for long durations of time, and does not wet the molten aluminum. The non-wetting characteristics are desirable for molten aluminum application. This is in contrast to coatings made from the first nickel-based binder alloy of which, when used alone, provides relatively good wetting characteristic. The combination of nickel and chromium in the metallic binders creates not only the weldability, hardness, and non-wetting characteristics, but also allows the creation of a flame oxidation layer on the exterior surface of the coating.

For purposes of this detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

In this application, the use of the singular includes the plural and the plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances.

Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims. 

1. A molten metal resistant composite coating comprising: a hard carbide phase; and a metallic binder comprising Ni and Cr.
 2. The molten metal resistant composite coating of claim 1, wherein the hard carbide phase comprises from 60 to 70 weight percent of the composite coating.
 3. The molten metal resistant composite coating of claim 1, wherein the carbide phase comprises tungsten carbide particles having an average size from 70 to 180 microns.
 4. The molten metal resistant composite coating of claim 1, wherein the metallic binder comprises from 30 to 40 weight percent of the composite coating.
 5. The molten metal resistant composite coating of claim 1, wherein the metallic binder comprises from 70 to 86 weight percent Ni.
 6. The molten metal resistant composite coating of claim 5, wherein the metallic binder further comprises from 7 to 20 weight percent Cr.
 7. The molten metal resistant composite coating of claim 1, wherein the metallic binder comprises 70 to 86 weight percent Ni, from 7 to 20 weight percent Cr, from 1 to 5 weight percent Si, from 2 to 5 weight percent Fe, from 1 to 4 weight percent B, and the balance incidental impurities, and has a hardness range of HRC from 33 to
 50. 8. The molten metal resistant composite coating of claim 1, further comprising an oxide surface layer over at least a portion of the base layer.
 9. The molten metal resistant composite coating of claim 8, wherein the oxide surface layer has a thickness of less than 50 microns.
 10. The molten metal resistant composite coating of claim 8, wherein the oxide surface layer is formed by flame oxidation and comprises oxides of the metallic binder metals.
 11. A tool capable of withstanding exposure to molten metal comprising: a substrate, and a composite coating over at least a portion of the substrate, wherein the composite coating comprises a hard carbide phase and a metal binder phase.
 12. The tool of claim 11, wherein the hard carbide phase comprises from 60 to 70 weight percent of the composite coating.
 13. The tool of claim 11, wherein the carbide phase comprises tungsten carbide particles having an average size from 70 to 180 microns.
 14. The tool of claim 11, wherein the metallic binder comprises from 30 to 40 weight percent of the composite coating.
 15. The tool of claim 11, wherein the metallic binder comprises from 70 to 86 weight percent Ni.
 16. The tool of claim 15, wherein the metallic binder further comprises from 7 to 20 weight percent Cr.
 17. The tool of claim 11, further comprising an oxide surface layer over at least a portion of the composite coating.
 18. A method of coating a substrate comprising: depositing a composite coating on at least a portion of the substrate, wherein the composite coating comprises a base layer comprising a hard carbide phase and a metallic binder phase.
 19. The method of claim 18, wherein the composite coating is deposited on the substrate by plasma transferred arc welding.
 20. The method of claim 18, further comprising forming an oxide surface layer over at least a portion of the base layer by flame-oxidation. 